The present invention relates to a method of and an arrangement for converting the temporal rate of high definition television pictures, as well as to a television picture decoder comprising such an arrangement.
There is no escape from the fact that future television programmer will very soon be filmed with a high definition. The D2 MAC/Packets standard which is the result of years of European effort improves the quality of television pictures to a considerable extent by obviating the inherent disadvantage of the frequency-division multiPlexing used in the SECAM standard, which is based on a fixed constant, e.g. a number of lines equal to 625, which precludes obtaining a picture resolution of high quality. The object of high definition television systems is to, horizontally and vertically, a spatial resolution which is substantially twice as good as the resolution provided by the D2 MAC/Packets standard while still using the analog channel provided in the MAC standard which has a capacity is limited to one quartz of the amount of information available in these high definition Pictures.
Encoders, used on transmission, and decoders, used on reception, have therefore been adapted to this high definition data volume on be transmitted to the available channel. The basic principle is to use a spatial-temporal decimation, which renders it possible to remove three signal samples out of four before transmission. It is a known fact that in a filmed scene, the sensitivity of the eye to spatial resolution decreases as the speed of motion increases, and vice versa. The spatial-temporal decimation is therefore accompanied, in accordance with the principle proposed, by an adaptation of said decimation intended to make the spectral contents three-dimensional in an as accurate manner as possible: to this end a modification of the type of signal sampling is made, depending on whether the processed picture zones (these pictures are divided into blocks) correspond to fixed picture portions, portions with little motion, or portions with much motion.
These principles are described in a detailed manner in the paper "HD-MAC coding of high definition television signals", F.W.P. Vzeeswijk, F. Fonsalas, T.I.P. Trew, C.C. Smith and M. Haghiri, presented during the exhibition IBC'88, Brighton (Great Britain), 23-27 Sep. 1988. Put more accurately, in accordance with the defined principle of utilizing three types of sub-sampling operations corresponding to three different transmission rates and usually designated in accordance with the duration of the corresponding refresh periods, which are here 80, 40 and 20 milliseconds, respectively, the picture, which is divided into blocks to ensure that the processing operations are adapted to the local content of the picture, is received in the encoder via three parallel-arranged branches each effecting one of the three sub-sampling operations in accordance with the range of motions. For each block, one of the three branches is chosen and the corresponding samples are conveyed for transmission in the analog form via the analog transmission channel used in the MAC standard, the decision of whether the 80, 40 and 20 ms branch is chosen being made by a decision circuit which is preceded by a circuit ;or estimating said motion. A digital assistance channel, of the Digitally Assisted TeleVision system ensures the transmission of the motion information components and the choice of the branch to the decoder arranged at the receiver end.
In a corresponding manner, this decoder comprises three branches which are also denoted by 80, 40 and 20 ms and which operate in a continuous and parallel manner. At each instant, thanks to the digital assistance data, the branch to be selected is known. Actually, since the three temporal rates defined in the foregoing are transmitted by timedivision multiplex with three different spatial-temPoral sampling operations, the current data can only feed the selected branch. After recovery of the data structures the output signals of the branch thus selected are supplied to constitute the high definition output of said decoder.
The referenced document also describes in detail the branch selection principle which is based on, for each block and for the three branches, a calculation of the distortion relative to the original picture and the selection, for transmission, of that one of the branches having the minimum distortion. This document also describes the motion estimating and compensating principle, providing the elimination of one field out of two fields before transmission, and the recovery of the missing fields, on reception, by using the existing temporal information between the two transmitted fields. This temporal information is most often simple translation information represented by a vector pointing in the direction of the motion. In a general way, the motion estimation and compensation thus effected make use of the redundancy of the temporal content of the picture sequences, by extracting before transmission auxiliary information components which are thereafter transmitted in parallel with the principal information components to enable the recovery of the pictures at the receiving end.
These various techniques result in pictures which are indeed of a high quality, but in which because of the 50 Hz field frequency, a jitter phenomenon may remain, more specifically on the picture zones of a very high brightness, which phenomenon is more perceptible on larger screen television set is larger, and consequently is very annoying to the eye.